Regenerative amplifier



June 27, 1933. H. NYQUlsT REGENERATIVE AMPLJFIER Filed May l, 1950 terms eed-ack rado al?! hase INVENTOR @WJ-@miga Patentedfilune 27, 1933 UNITED STATES PATENT OFFICE HARRY NYQUIST, OF MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPI-I COMPANY, A CORPORATION OF NEW YORK REGENERATIVE AMPLIFIER Application led- May 1, 1930. Serial No. 449,037.

This invention relates to regenerative amplitiers and particularly to an amplifier that will remain stable even though the feed-back ratio is greater than unity and the current fed back is in phase at one or more frequencies. i

In certain forms of electrical circuits cmploying vacuum tubes a portionof the out put current is fed back to the input either unintentionally or by design. Such a device may be looked upon as an amplifier Whose output is connected to the input through a transducer. When thus connected the resulting combination may be either stable or unstable. The circuit is said to be stable yvhen an impressed small disturbance yvhich itself dies out, results in a response which dies out. It is said to be unstable when such disturbance results in a response vwhich goes on indefinitely, either staying at arelatiyely small value or increasing until it is limited by the nonlinearity of the amplifier. Heretofore it has been understood if a regenerative circuit had a feed-back ratio greater than unity and the current fed back was in phase with the input current, that such regenerative circuit Was unstable. I have recently discovered'thatat is possible to have a stable regenerative circuit even though the feed-back ratio is greater than unity and the current fed back is m phase at one or more frequencies with the 1nput current.

My invention resides 1n a method for determining the stability of a regenerative 5 amplifier, and also resides in a regenerative amplifier that is stable even though the feedback ratio is greater than unity and in spitey of the fact that the current fed back at one or more frequencies may be in phase With the input current.

This invention Will be understood from the following description when read in connection with the attached drawing of which Figure 1 shows schematically a regenerative aniplifier embodying my invention, Fig. 2 1llustrates graphically the'method employed in determining the stability of such amplifiers, Fig. 3 illustrates a simple method for obtaining the feed-back ratio, and Fig. 4 is a vector diagram usedin the description of Fig. 3.

1n Fig. 1 the input current is impressed upon the amplifier by the transformer 1 and the output current is impressed by the transformer 2 upon the output circuit. The secondary of the transformer 1 is connected across the input of the vacuum tube 3 of the first stage of the amplifier. The input circuit of tube 3 includes also the resistances 4, 5 and 6, the latter being shunted by the condenser 7. The output circuit of tube 3 is coupled by the condenser 8 with the input circuit of the tube 9 constituting the second stage of the amplifier. The condenser 8 is shunted by a resistance 10, the purpose of which will be made clear hereinafter. Connected between the condenser 8 and the grid of the tube 9 are the inductances 11, 12 and 13, which together With the coiidensers 14, 15

and 16, connected in shunt across the input circuit, constitute a low-pass filter. Each of the inductances is shunted by one of the resistances 17, 18 and 19, and each of the condensers has connected in series therewith one of the resistances 20, 21 and 22. The purpose of the resistances is to prevent undesired phase shifts at certain frequencies, thc manner of doing which ivill later be described. The output of tube 9 is coupled by condenser 23 with the input of tube 24, constituting the third stage of the amplifier. Condenser 23 is shunted by a resistance 25 which is similar to resistance 10. Connected across the output of tube 24 are the resistances 26 and 27. A feed-back circuit comprising the conductors 28 and 9, the condenser 30 and resistance 31, extends from the junction point of the resistances 26 and 27 in the output circuit of the tube 24 to the junction point of the resistances 5 and 6 in the input circuit of the tube 3. Each of the tubes has associated therewith sources of current or potential to energize the filament and to supply the requisite plate and grid voltages to insure the proper functioning of the amplifier.

The invention relates to the type of amplifiers in which the current is fed back in phase opposition. This results in improved repeater characteristics, particularly in freedom from modulation. The general aim is to feed back as much current as possible within the working range of frequencies, without producing an unstable condition outside of the vworking range where the phase relations may be more unfavorable than within the working range. In the design of a regenerative amplifier without transformers in the regenerative portion of the circuit it is necessary to employ an odd number of vacuum tubes for the reason that a vacuum tube has the property of changing the phase of the current passing therethrough. An odd number of tubes will therefore tend to insure that the current fed back will be in phase opposition to the input current. Owing to the presence in the circuit of a certain amount of reactance the current fed back will not be in phase opposition at all frequencies to the /f t Y there is another point designated f., where the input current.

Fig. 2 shows the manner in which the feedback varies with frequency both as to magnitude and phase angle. The curve shown in that figure represents the locus of the points corresponding to the feed-back ratio and phase angle for successive values o f frequency from zero to infinity. The distance from any point on the curve to the origin represents the feed-back ratio and the angle that that line makes with the positive axis represents the phase shift. The vrelativec magnitude of the feed-back ratio may be determined by comparison with the distance from the origin of the point 1 upon the ositive axis which point represents unity eedback ratio of zero angle. The feed-back ratio is usually a complex quantity. It denotes the ratio by which the current or voltage is attenuated by one round trip vthrough the circuit, including` the phase shift. A simple method for obtaining the feed-back ratio is illustrated by Fig. 3 which is a modification of that part of Fig. 1 between AA and BB. The circuit of Fig. 1 is opened at some convenient place, such as next to the grid resistance 32 of the third stage. A voltage V1 is applied to this resistance by an oscillator l'35 and its magnitude measured by the meter 39 by connect/ing to the jacks 38. rI he elements 33 and 34 are adjusted to simulate the impedance ofthe resistance 32 in arallel with the grid of the tube 24, so that t e voltage V2 which is next measured on jacks 37 shall not be affected by improper termination. The voltage V3 is measured on jacks 36. A triangle may be constructed having the magnitudes V1, V2, V3 as its sides, as in Fig. 4, in order to determine the angular displacement, 9, between V1 and V2. This angle and the ratio Vg/V1 give the complex feed-back ratio. Such measurements afford the basis for deriving the data which when plotted will produce the feed-back characteristic of the amplifier. Now let a straight line be drawn from the point numbered 1 to point a on the feed-back characteristic. Let a be moved along the curve while the line l-a changes length and rotates about point 1 as a pivot. I have discovered that when the line 1-a rotates through a total of zero degrees as fa follows the course of the feed-back characteristic from f=0 to f=oo, the circuit upon which that characteristic curve is based will be stable even though the feed-back ratio is greater than unity, and has zero angle at certain frequencies. For exam le, in the curve shown in Fig. 2, the point esignated f3 represents a feed-back ratio in which the current fed back is not only in phase with the input current, but the ratio is slightly under 7. Following the curve further toward increasing frequency, we find another point f4 where the Same conditions prevail. Subsequently,

current fed back is in phase with the input current, but the ratio of output to input is less than unity. Since the line 1-a rotates through a total of .zero degrees as a follows the entire curve representing the feed-back characteristic o f the circuit shown in Fig. 1, the circuit will be stable in spite of the fact that at the frequencies a and f., the ratio is greater than unity, an the output currents are in phase with the input currents. It is desirable to point out, however, that since the point f5 lies close to the point l representing unity feedback, the curve might be caused to extend to the right so that f5 lies to the right of the point 1 if the gain of the amplifier should be greatly increase above that represented by the curve of Fig. 2. In this case the line l--a would rotate through 360 as a follows the curve from f==0 to f= oo and the circuit would be unstable.

In the curve shown in Fig. 2 the working range of frequencies is represented by that portion lying between f1 and f2. vThe remaining portions of the curve below f1 and f2 must be considered from the standpoint of the possibility of feed-back at frequencies above or below the working range. In order to maintain stability of the regenerative amplifier its characteristic curve must be kept away from the point representing unity feedback. It willbe seen from Fig. 2 that that portion of the curve representing frequencies below f1 approaches the positive axis from the third quadrant (the. quadrants being designated I, II, III and IV) which tends to insure that the curve will not enclose the point of unity feed-back. The performance `of the curve below f1 is due to the presence resistances 10, 25 and 31, and the tendency of the curve to extend into the first quadrant and to enclose the point 1 is prevented.

The behavior of the curve in the range from f2 to f., and for some distance beyond, is caused by the presence of the network that is located between the first and second stages of the amplifier which network resembles a low-pass filter. If the filter structure comprised merely the inductances 11, 12 and 13, and the condensers 14, 15 and 16, there would be a continuous variation in the phase angle with frequency, so that each element of the filter would produce a phase shift approaching 90 degrees at very high frequencies. That would cause the curve to surround the point 1 and produce a condition of instability. By connecting the resistances 17, 18 and 19 in shunt with their corresponding inductances,

and also by connecting the resistances 20, 21"

and 22 in series with their corresponding condensers 14, 15 and 16, the phase shift at very high frequencies is zero, and consequently, there results a condition such as is shown in Fig. 2 in the neighborhood of f4. The performance of the curves at frequencies around f5 and at higher frequencies is caused principally by distributed capacity. This effect cannot be eliminated by devices used in connection with the low-pass filter, and therefore for those very high frequencies the curve will surround the origin. It is desirable to point out that it is inevitable to have the condition shown at f5 which may result in singing if the gain is increased sufiiciently. However, a low-pass filter having the characteristics hereinbefore described, introduces great losses at the frequency f5 and therefore postpones the condition of instability until the gain of the amplifier is made very large. It is to be understood, however, that the number of sections of the filter may be increased indefinitely.

It is usually considered preferable, in regenerative amplifiers of this sort, to make the resistances in the plate circuit of the last tube form a Wheatstone bridge with the plate impedance so that the feed-back will not be affected by changes in the impedance connected to the output transformer. The elements 5, 6 and 7 form part of a similar bridge at the input.

While this invention has been disclosed as embodied in a certain arrangement, it is to be understood that such arrangement is purely schematic and that the invention is capable of embodiment in other and different forms without departing from the spirit and scope of the appended claims.

That is claimed is:

1. A regenerative amplifier comprising a network including reactances made up of series and shunt elements, the series reactance elements having resistances in shunt therewith and the shunt reactance elements having resistances in series therewith, an amplifying element connected to said network, and a feed-back connection between said amplifying element and said network.

2. A regenerative amplifier comprising a low-pass filter including coils and condensers in series and in shunt respectively, the series elements having resistances in shunt therewith and the shuntelements having resistances in series therewith, and amplifying element connected to said filter, and a feed-back connection between said amplifying element and said filter.

3. A regenerative amplifier comprising al plurality of amplifying elements, a condenser shunted by a resistance connected in series between ad]acent amplifying elements, a low-pass filt'er including series inductance coils and shunt condensers connected between two of said amplifying elements, each of said coils having a resistance in shunt therewith, and each shunt condenser having a resistance in series therewith, and a feed-back connection between the output of the last amplifying element and the input of the first amplifying element.

4. A circuit as defined by claim 1 in which the parts are so proportioned-that the locus of the points representing magnitude and phase angle of the feed-back ratio for frequencies between zero and infinity shall be a curve such that a straight line drawn from any point on this curve to the point representing unity feed-back ratio and Zero angle shall generate a total angle of zero as the straight line rotates about the said unity point as a pivot with its other end following the curve from zero to infinite frequency.

5. A regenerative amplifier comprising an odd number of vacuum tubes, a condenser shunted by a resistance in series between adjacent tubes, a low-pass filter including series` inductances and shunt condensers connected between two of said tubes, each of said inductances having a resistance in shunt therewith, and each shunt condenser having a resistance in series therewith, anda feed-back connection including a condenser shunted by a resistance between the output of the last valuum tube and the input of the 'first vacuum tu e. 6. A regenerative amplifier comprising an odd number greater than unity of vacuum tubes, means coupling the output of each tube to the input of the next succeeding tube, a feed-back circuit connecting the out-put of the last tube to the input of the first tube, the said coupling means between the amplifier stages including means to control the shifting of the phase of frequencies lower than those in the working range of frequencies, and also including means to control the shifting of the phase of the frequencies higher than nose in the said working range.

7. A regenerative am lifier comprising a plurality of vacuum tu es, means coupling the output of each tube to the input of the next succeeding tube, a feed-back circuit connecting the output of the last tube to the inut of the first tube, the said coupling means etween the amplifier stages including means to control shifting of the phase of frequencies lower than those in the working range of frequencies, and also includingr means to control the shifting of the phase of frequencies higher than those in the said working range, in such manner that stability of operation is attained with a feed-back ratio that for some frequencies is greater than unity with zero phase shift.

8. The method of stabilizing a regenerative amplifier which consists in adjusting the constants of said amplifier for a given frequency and measuring the feed-back ratio and thc phase angle for that frequency, then adjusting said amplifier for another frequency and continuing through a sufficient range of frequencies so that a curve can be drawn from zero to infinity, plotting the resulting measurements, and then adjusting the constants of the amplifier until the locus of the measurements is aeclosed curve that excludes the point corresponding to unity feedback ratio and zero angle.

9. The method of stabilizing a regenerative amplifier which consists in opening the circuit atsome point, measuring the input and output currents or voltages and their phase difference, plotting the points representing the magnitude and phase angle of the feed-back ratio and drawing the result ing locus of said points from zero to infinite frequency, and adjusting the constants of the regenerative amplifier until the resulting locus of said points constitutes a curve such that a straight line of variable length, rotating with one end as a pivot on the point corresponding to unity feed-back of zero angle, with the other end of the line tracing the said locus curve from zero to infinite frequency shall rotate through a total angle equal to zero.

10. The method of stabilizing a regenerative amplifier which consists in measuring and plotting the feed-back ratio and phase angle for a sufficient range of frequencies, and drawing their locus from zero to infinite frequency, adjusting the constants of the amplifier until the locus has such a path that a point traveling over the locus from zero to infinite frequency, and always joined by a straight line to the point of unity feedback ratio and zero angle as a pivot, will cause said line to turn through a total of zero. j

In testimony whereof, I have signed my name to this. specification this 30th day of April, 1930.

HARRY NYQUIST. 

